The invention is directed to an alkaline cyanide bath for the electrolytic deposition of brilliant copper-tin-alloy coatings, especially copper-tin alloy coatings having 45 to 60 percent copper, consisting of (or consisting essentially of) 1 to 60 g/l of copper in the form of copper cyanide, 7 to 30 g/l of tin in the form of stannate, e.g., alkali stannate such as sodium stannate or potassium stannate, 0.1 to 100 g/l of one or more complex formers of the group consisting of phosphates, e.g., sodium phosphate or potassium phosphate, polyphosphates, e.g., sodium polyphosphate, or potassium polyphosphate, phosphonates, e.g., disodium methylenediphosphonate, disodium hydroxymethanediphosphonate, disodium hydroxyethanediphosphonate, and polyhydroxy carboxylic acids and salts thereof, e.g., tartaric acid, citric acid, gluconic acid, and their sodium and potassium salts, 1 to 50 g/l of free alkali cyanide, e.g., sodium cyanide or potassium cyanide, 1 to 50 g/l of free alkali hydroxide, e.g., sodium hydroxide or potassium hydroxide, and 0 to 50 g/l of alkali carbonate, e.g., sodium carbonate or potassium carbonate.
The coating can be deposited, for example, on steel.
It has been known for many years to deposit copper-tin alloy coatings from electrolytic baths. There have especially been used coatings which contain 45 to 60 percent copper, preferably 55 to 60 percent copper, since these have a clear silver brightness and is not inclined to tarnish. Therefore, they are used in the decorative electroplating as replacements for, for example, silver, chromium, or aluminum. However, copper-tin alloy coatings also find increasing industrial use because of their very good soldering properties, their resistance to abrasion, and their low electrical contact resistance.
Such copper-tin alloys were predominantly deposited from alkaline, cyanide containing electrolysis baths which contain the tin as stannate. Other electrolysis baths contain phosphate and pyrophosphate as complex former and also colloids, such as e.g., polypeptides as brighteners (German OS 860 300). These known baths must be operated at high, constant temperatures (65.degree. C. and higher) in order to obtain uniform layers of constant composition. The same is true also for the adjustment of the cyanide and hydroxide concentration in the bath. Therefore, working with these baths is difficult and cumbersome.
Therefore, it was the problem of the present invention to develop an alkaline cyanide bath for the electrolytic deposition of bright to brilliant copper-tin-alloy coatings, especially copper-tin alloy coatings having 45 to 60 percent copper, consisting of (or consisting essentially of) 1 to 60 g/l of copper in the form of copper cyanide, 7 to 30 g/l of tin in the form of stannate, e.g., alkali stannate such as sodium stannate or potassium stannate, 0.1 to 100 g/l of one or more complex formers of the group consisting of phosphates, e.g., sodium phosphate or potassium phosphate, polyphosphates, e.g., sodium polyphosphate, or potassium polyphosphate, phosphonates, e.g., disodium methylenediphosphonate, disodium hydroxymethanediphosphonate, disodium hydroxyethanediphosphonate, and polyhydroxy carboxylic acids, e.g., tartaric acid, citric acid, gluconic acid, 1 to 50 g/l of free alkali hydroxide, e.g., sodium hydroxide or potassium hydroxide, and 0 to 50 g/l of alkali carbonate, e.g., sodium carbonate or potassium carbonate which can be operated at lower temperatures and in which the coating composition is less strongly dependent upon the deviations of the bath components.